1. Field of the Invention
The present invention relates to the field of flat panel display screens. More specifically, the present invention relates to the field of flat panel field emission displays (FEDs).
2. Related Art
Flat panel FED display devices, also called thin CRTs, include a flat panel screen having rows and columns of intersecting lines. The lines intersect to form subpixels. In the past, amplitude modulated (AM) voltage signals have been applied to the column lines in order to display images on the FED screen. Pulse width modulation (PWM) has been proposed for gray scale generation. In low-capacitance panels and in low gray scale generation situations, this technique may have application. However, in high capacitance (e.g., about 1 pf/subpixel) panels and 256 gray scale situations, PWM is not useful and in the general case consumes too much power. In these situations, PWM has not been used generally for carrying the image data to the columns because of certain time restraints inherent in the FED screen display technique. For instance, in order to provide 256 different gray scale levels, the time period allotted for each column driver must be divided into at least 256 discrete time intervals. Assuming a display having 400 rows, the 1/60 sec frame update rate is divided by 400 yielding a 1/24,000 sec time frame in which all columns need to be updated.
However, there are many columns that must be updated during this time period. This leads to a less than 60 ns pulse width (per column) to support, for instance, the XGA video format. Also, with less than 5 ns rise times for typical driver circuitry, the drivers and displays have less than 10 ohm impedance paths. These requirements unfortunately prohibit PWM gray scale with 1 pf/subpixel displays because it is not practical to divide the small column drive time into the many discrete time intervals required for PWM given the limited rise and fall times of the driver circuits. In addition, PWM increases the CV.sup.2 losses over the AM technique in most cases.
In the field of flat panel display devices, it is often necessary to adjust the brightness of the display screen. AM has been used in the past to adjust the brightness of a FED device. However, with AM, the operating point of the emission cathode is changed thereby possibly affecting the gamma (e.g., v-i characteristics of the subpixels) and white point balance of the display screen. It would be desirable to provide a brightness adjustment that did not affect the gamma or white balance of the display screen.
Active matrix liquid crystal devices (AMLCDs) typically contain one or more backlighting lamps that project light through the active matrix of liquid crystal cells. The brightness adjustment of AMLCD devices alters the gray scale resolution of the pixels. These flat panel display screens alter the brightness of the display by controlling the electrical drive to, and hence the intensity of, the backlighting lamp. However, by its nature, the color and the uniformity produced by an AMLCD device degrade as the backlighting lamp is moved away from an optimum brightness point. The optimum brightness point is typically factory set. By altering the gray scale resolution of the pixels when performing brightness adjustment, this prior art method of altering the brightness of a flat panel display has the unfortunate side effect of degrading the quality of the displayed image. It is desirable to provide a brightness adjustment for a flat panel display screen that does not compromise the gray scale quality of the pixels.
In another prior art mechanism for altering the brightness of an AMLCD, the image data used to render an image on the screen is altered as it is fed to the display. A function composed of a gain and an offset value is programmed into the display and all image data is then passed through the function which multiplies the data by the gain value and then adds the programmed offset value. The values of the above function are then altered as the brightness needs to be increased or decreased. This prior art mechanism for altering screen brightness is disadvantageous because it requires relatively complex circuitry for altering the large volume of image data. Secondly, this prior art mechanism degrades the gray scale quality of the image by altering the gray scale resolution of the flat panel display. It is desirable to provide a brightness adjustment for a flat panel display screen that does not alter the image data nor compromise the gray scale resolution of the image.
Flat panel field emission displays (FEDs) do not use backlighting lamps. Flat panel FEDs utilize emitters each having an anode and a cathode and a gate. The voltage applied across an individual emitter (gate to cathode) causes it to release electrons toward a phosphor spot located on a display screen. Many emitters are associated with a single phosphor spot. A pixel is composed of three (e.g., red, green and blue) independently controlled phosphor spots. The gray scale content of a pixel within a flat panel FED screen is represented by the voltages applied to the red, green and blue emitters that constitute the pixel. However, a brightness adjustment mechanism that alters the relative voltages applied to the emitters of the red, green and blue phosphor spots will vary the gray scale quality of the pixels within a flat panel FED screen. It is desirable to provide a brightness adjustment for a flat panel FED screen that does not compromise the gray scale resolution of the pixels.
One prior art mechanism for altering the brightness of an FED alters the high voltage (e.g., several kilovolts) applied to the emitter's anode. This method is disadvantageous because it requires a variable output high voltage power supply which is more complex and hence more expensive than a constant voltage output power supply. Secondly, this prior art mechanism requires that the brightness adjustment circuitry be implemented with high voltage components rather than less expensive, simpler low voltage components. It is desirable to provide a brightness adjustment for a flat panel FED screen that does not require altering high voltage levels nor that requires high voltage components.
Accordingly, the present invention provides a mechanism and method for controlling the brightness of a flat panel display screen that does not compromise the gray scale resolution of the pixels of the display screen. The present invention also provides a mechanism for altering the brightness of a flat panel display screen that does not alter the image data. The present invention also provides a mechanism for altering the brightness of a flat panel display screen that does not alter the gamma or white balance of the display screen. These and other advantages of the present invention not specifically mentioned above will become clear within discussions of the present invention presented herein.